The size of an electric rotating machine depends on its nominal torque. The higher the torque that a motor is capable of delivering, the bigger the electric motor, all other things being equal. There are however applications for which it is desirable to achieve at the same time considerable powers and a large degree of compactness of the motor. Simply to give a practical example, when it is desired to implant electric traction motors in the wheels of motor vehicles, it is desirable to be able to develop powers of at least 10 kW per motor, and even most of the time at least 25 or 30 kW per motor, for the lowest possible weight in order to limit as much as possible the unsuspended weights. It is also desirable that the space requirement is extremely small, exceeding by as little as possible the internal volume of the wheel so as not to interfere with the elements of the vehicle during travels of suspension and during other types of movement of the wheel relative to the body shell of the vehicle.
These two imperatives (high power, low space requirement and weight) make it very problematical to install electric traction motors in the wheels of passenger vehicles without radically improving the weight/power ratio of the electric machines currently available on the market.
Choosing a high speed for an electric motor when the motor is designed is a solution making it possible, for a given power, to reduce the torque and hence the space requirement. In other words, for a given nominal power of the motor, the higher its nominal rotation speed, the smaller its space requirement will be.
Raising the rotation speed of an electric rotating machine on the other hand poses many problems, notably with respect to the centrifugal forces sustained by the elements of the rotor, in particular the magnets.
The (mechanical and acoustic) vibrations are also a difficulty that increases as the rotation speed increases.
A specific design for achieving high rotation speeds has already been proposed in patent application EP 1001507. The speeds proposed in this patent application are of the order of 12 000 rpm, by proposing for this a particular arrangement of the assembly consisting of a polygonal one-piece shaft and polar parts judiciously placed around this shaft.
An enhancement making it possible to aim at speeds of the order of 20 000 rpm has been proposed in patent application EP 1359657 by proposing for this an arrangement using wedges to radially lock the magnets in their housings.